Distributional Ecology and Behavioral Thermoregulation of Fishes in Relation to Heated Effluent from a Power Plant at Lake Monona, Wisconsin

Abstract

Cooling water for a 220-megawatt power plant is pumped from 5 m beneath Lake Mononaˈs surface, heated about 10 C, and discharged into the littoral zone. Maximum temperatures in the effluent-outfall area approach 35 C in summer and 14 C in winter; unheated parts of the littoral zone rarely exceed 29 C. To assess distributional responses of fishes to operation of the power plant, the outfall area and two reference areas were electrofished on 30 dates between 8 August 1968 and 21 August 1969. Some fishes avoided the outfall area; these were Perca flavescens, Cottus bairdi, subadult Morone mississippiensis, and subadult Ictalurus melas. Others were occasionally or usually concentrated in the outfall area relative to reference areas—Lepisosteus osseus, adult Cyprinus carpio, adult M. mississippiensis, young Lepomis gibbosus, L. macrochirus, and Micropterus salmoides. The tendency for fish of a given species and size to concentrate in or avoid the outfall area generally did not reverse from season to season or from day to night. Any differences in growth rate of young fishes among outfall and reference areas were obscured by size-dependent movements of the fish. In August 1970, body temperature of fishes in the outfall area varied with species and time of day. During afternoon, median temperatures of Cyprinus carpio, Lepomis gibbosus, L. macrochirus, and Micropterus salmoides were higher than 29 C, while those of Morone mississippiensis, Ambloplites rupestris, Pomoxis nigromaculatus, and Perca flavescens were lower than 29 C. At night, only Lepisosteus osseus and C. carpio had median body temperatures higher than 29 C. Young Lepomis macrochirus collected during summer 1970 from the outfall area had estimated acclimation temperatures between 29.4 and 31.3 C, even though temperatures between 25 and 36 C were available. To have had acclimations between 29.4 and 31.3 C, the fish must have avoided exposure to temperatures higher than about 31.5 C. This conclusion agreed substantially with the distribution of body temperatures of young L. macrochirus; the highest body temperature among 31 specimens was 31.8 C. Temperatures at which fishes lived in the outfall area during summer 1970 were positively correlated (p < 0.01) with the degree to which they were concentrated in the outfall area during the two previous summers. The strength of the correlation argued that, during summer, temperature was the primary factor governing not only distribution of fishes within the outfall area, but also the abundances of fishes in the outfall area relative to unheated parts of the littoral zone. Moreover, integration of field and laboratory results suggested that fishes were distributed within the outfall area according to their different temperature preferenda. Preferred temperatures of six Lake Monona fishes were measured by allowing each of several specimens to behaviorally regulate the temperature of its tank. Midpoint of the preferred temperature range agreed well with the median body temperature recorded in the outfall area during afternoon (considering, in the case of Perca flavescens, that temperatures below 26.5 C were not available in the outfall area): Cyprinus carpio, 31.8 C (laboratory), 30.6 C (field); Lepomis macrochirus, 30.3, 29.4; Micropterus salmoides, 29.1, 29.7; Pomoxis nigromaculatus, 28.3, 28.3; A. rupestris, 27.3, 27.5; and Perca flavescens, 23.4, 27.1. In other laboratory experiments, Lepomis macrochirus and Perca flavescens were required to partition time between an environment with preferred temperature and an environment with food but either warmer or cooler than the preferred temperature. Each species made forays for food into water with extreme temperatures. However, neither laboratory nor field results suggested that thermoregulatory behavior was overridden by feeding behavior, even though zooplanktonic food was more abundant in the outfall area than in unheated parts of the littoral zone.